A key challenge, and also a main research focus in the development and application of wide band gap technology, particularly gallium nitride (GaN), is the lack of high quality, cost effective and low-defect materials. Due to a lack of a commercially available low-defect GaN substrate material, GaN is commonly grown on substrates such as sapphire, silicon carbide, zinc oxide, silicon, etc. High densities of threading dislocation ensue due to lattice and thermal mismatches with these substrates. Lateral epitaxial overgrowth, pendeo-epitaxy, and bulk homo-epitaxial growth techniques have been proposed to improve the quality of GaN material, yet none of these techniques are currently optimal. What is desired is a fabrication technique capable of producing high-quality, low-defect gallium nitride at a reasonable cost.
In light emitting sources, such as light-emitting diodes (LEDs), the emission wavelength of light depends on the bandgap of the compound semiconductor. Currently, white or “mixed colored” LEDs are obtained by combining a blue LED with a yellow phosphor. The problems associated with this approach are low emission efficiency, Stokes losses, device lifetime, and complex packaging (because a phosphor layer must be manufactured into the device). Another approach to making white or mixed colored LEDs is to assemble numerous monochromatic red, blue and green LEDs. This approach is very expensive and complex since each of the blue, green and red LEDs have to be addressed independently and a feedback mechanism is needed. Thus, there is a need for a simple monolithic white LED device.
For several years, researchers have touted the utility of quantum dot laser diodes (QDLDs) over standard quantum well laser diodes (QWLDs) (Arakawa et al., Appl. Phys. Lett., 40, 939–941 (1982)). Due to the narrow delta-like density of states associated with quantum dots (QDs), lower threshold currents are required for lasing. Further, QDs can alleviate the deleterious effects of unintentional defects in the active region, due to their exceptional carrier capture cross-sections. However, current attempts to capitalize on QDs for lasing applications are limited by the inability to form uniform, monodisperse dots throughout a film. While monodispersion is often not a concern for LEDs, it is especially important for laser diodes (LDs), where the generation of coherent, single phase light is essential. Thus, a more practical method of generating uniform, monodisperse QDs would be desirable.
Quantum confinement and other surface effects influence the emission wavelength when the structural length scale reaches the nano-regime. Hence, for a given material composition, different emission wavelengths across the entire electromagnetic spectrum can be achieved by varying the size of the nano-structural features (e.g., CdSe, AlGaN, InP, J. Phys. Chem. B, 101, pp. 4904–4912, 1997; J. Am. Chem. Soc., 115, pp. 8706, 1993; Appl. Phys. Lett., 75, pp. 962–965 1999; J. Korean Phy. Society, 39, p. 141, 2000). It has also been recognized that nanostructures such as quantum dots and quantum wells are key components of light emitting sources. In conventional wide bandgap based LEDs, these nanostructures are formed naturally and exhibit broad size and composition distributions. Control over the uniformity (both size and composition) of these nanostructures in the active layer of the LED devices remains a major challenge, however. Though some researchers have reported “white” electroluminescence using disperse self-assembled InGaN quantum dots (Jpn. J. Appl. Phys., 40. pp. L918–L920, 2001), these were not, however, made by a templated growth method. The device itself was based on multiple quantum wells, and the internal quantum efficiencies were severely limited by size disorder and spatial non-uniformity. Efficient monolithic white LEDs will require intelligent nanostructuring (e.g., nanolithography) of the quantum confined active region.
Defect reduction through strain relief inside 3-dimensional (3-D) confined nanostructures grown on lattice-mismatched substrates has not yet been attempted based upon a systematic size and shape optimization via physical modeling of the phenomena of dislocation nucleation. Related art is restricted to defect reduction in substrate mismatched wide bandgap thin films and 2-dimensional structures using techniques such as lateral epitaxial overgrowth (U.S. patent application Ser. No. 09/780,069), pendeo-epitaxy (U.S. patent application Ser. No. 09/780,715), nano-lateral epitaxial overgrowth (U.S. patent application Ser. No. 10/273,926), and nano-columnar overgrowth (World Intellectual Property Organization Application Publication No. WO 02/44444 A1) among others.
Cuomo et al. (International Patent Application No. PCT/US01/44992) have demonstrated the formation of nanoscale columnar structures using a sputtering transport technique. This method has been shown to improve GaN epitaxy by alleviating thermal strain between the patterned substrate and the GaN film. However, this technique does not generate low-defect GaN because the sputtered structures cannot be controllably ordered and made monodisperse. Further, the techniques used to generate the nano-columns rely on sputtering conditions and not nanolithography patterning.
Bawandi et al. (U.S. Pat. No. 6,501,091 B1) have proposed a method for the fabrication of quantum dot white and colored light emitting diodes. The technology proposed by these authors is based on the colloidal synthesis of group II–VI semiconductors, such as CdSe and CdS, and their tailored dispersion in a transparent host matrix. The size and distribution of the quantum dots is chosen to emit light of monochromatic or white color. This method, however, is very different from the present invention since the particular LED device proposed by Bawandi et al. must be activated (illuminated) by another LED of shorter wavelength.
Lieber et al. (U.S. patent application Ser. No. 10/196,337) have proposed methods of growing semiconductor nanostructures (particularly, III–V semiconductor nanorods and nanotubes, such as GaN) that emit light. The nanorods can be further doped (n- or p-type), and p-n junctions within the same nanorod can be made. Furthermore, the rods can be addressed electrically and LED devices can be fabricated. However, Lieber et al. have only succeeded in making monochromatic (single wavelength) LEDs.
Block co-polymers have been used for the templated growth of monodisperse metallic quantum dots and nanowires (Science, 290, p. 2126, 2000). Block co-polymers have also been used for the templated growth of monodisperse semiconductor quantum dots, in particular AlGaAs on a GaAs substrate (Appl. Phys. Lett., 76, p. 1689, 2000). However, InGaAs and GaAs have very similar lattice parameters. To date, however, block co-polymers (or other lithographic techniques) have not been used for the templated growth of quantum dots (or other nanostructures) that present a large lattice mismatch with the corresponding substrate.
What is desired is a method capable of producing large and dense arrays of low-defect nanostructures (e.g., quantum dots) with arbitrary size and spacing on substrates that may or may not have a large lattice mismatch with the nanostructured material, and to be able to use such arrays in light emitting devices.